Definition:Convergent Sequence

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Definition

Topological Space

Let $T = \left({S, \tau}\right)$ be a topological space.

Let $\left \langle {x_n} \right \rangle_{n \in \N}$ be an infinite sequence in $S$.


Then $\left \langle {x_n} \right \rangle$ converges to the limit $\alpha \in S$ if and only if:

$\forall U \in \tau: \alpha \in U \implies \left({\exists N \in \R_{>0}: \forall n \in \N: n > N \implies x_n \in U}\right)$


Metric Space

Let $M = \left({A, d}\right)$ be a metric space or a pseudometric space.

Let $\sequence {x_k}$ be a sequence in $A$.

$\sequence {x_k}$ converges to the limit $l \in A$ if and only if:

$\forall \epsilon \in \R_{>0}: \exists N \in \R_{>0}: \forall n \in \N: n > N \implies \map d {x_n, l} < \epsilon$


For other equivalent definitions of a convergent sequence in a Metric Space see: Definition:Convergent Sequence in Metric Space


Normed Division Ring

Let $\struct {R, \norm {\, \cdot \,} }$ be a normed division ring.

Let $\sequence {x_n} $ be a sequence in $R$.


The sequence $\sequence {x_n}$ converges to $x \in R$ in the norm $\norm {\, \cdot \,}$ if and only if:

$\forall \epsilon \in \R_{>0}: \exists N \in \R_{>0}: \forall n \in \N: n > N \implies \norm {x_n - x} < \epsilon$



Standard Number Fields

When $A$ is one of the standard number fields $\Q, \R, \C$, and the metric $d$ is the usual (Euclidean) metric, the condition on convergence becomes:


Real Numbers

Let $\sequence {x_k}$ be a sequence in $\R$.

The sequence $\sequence {x_k}$ converges to the limit $l \in \R$ if and only if:

$\forall \epsilon \in \R_{>0}: \exists N \in \R_{>0}: n > N \implies \size {x_n - l} < \epsilon$

where $\size x$ denotes the absolute value of $x$.


Rational Numbers

Let $\sequence {x_k}$ be a sequence in $\Q$.

$\sequence {x_k}$ converges to the limit $l \in \R$ if and only if:

$\forall \epsilon \in \R_{>0}: \exists N \in \R_{>0}: n > N \implies \size {x_n - l} < \epsilon$

where $\size x$ is the absolute value of $x$.


Complex Numbers

Let $\sequence {z_k}$ be a sequence in $\C$.


$\sequence {z_k}$ converges to the limit $c \in \C$ if and only if:

$\forall \epsilon \in \R_{>0}: \exists N \in \R: n > N \implies \cmod {z_n - c} < \epsilon$

where $\cmod z$ denotes the modulus of $z$.



Also known as

A convergent sequence is also known by its variant converging sequence.

In the interests of consistency, the latter term is avoided on $\mathsf{Pr} \infty \mathsf{fWiki}$.


Also see